Currently, Traffic Collision Avoidance System (TCAS) bearing is determined from a TCAS interrogation response signal, by a 4-element antenna array. The elements are spaced equally about the circumference of the antenna array, geometrically at 90-degree intervals. The relative bearing is determined by measuring the phase difference of the response signal between opposite element pairs.
The length between opposite elements is short (within ½ wave length of the 1090 MHz XPDR signal), thus the TCAS bearing measurements are vulnerable to noise and susceptible to coupling. Thus, TCAS II (recommends evasive maneuvers) bearing measurement can be inaccurate. Usually, the error is no more than 5 degrees but it can be greater than 30 degrees. The low bearing accuracy introduces a large uncertainty in the TCAS position that increases with range. Thus, a displayed TCAS target symbol can appear to jump, due to these errors/inaccuracies.